In a timepiece, power generated by, for example, a mainspring or a motor is transferred to a hand through a wheel train to drive the hand. The wheel train is configured by engaging transmission wheels such as a second wheel and a third gear. In each transmission wheel, a gear and a pinion are coaxially integrated. Specifically, a hole into which the pinion is fitted is formed in the center of the gear, and the gear and the pinion are integrated by pressing the pinion into the hole of the gear along a shaft center direction. When both the gear and the pinion are made from metal, the peripheral portion of the hole of the gear and the pinion elastically deform. Therefore, it is possible to press the pinion into the hole.
In recent years, a gear made from a brittle material such as silicon has been tested so as to reduce its weight and simplify its shape. The brittle material may damage the gear when the pinion is pressed into the gear in the shaft center direction similar to the metal gear and pinion because the brittle material has an extremely small deformation volume. For this reason, a technique of fixing a pinion inserted into a hole has been proposed (see, e.g., Patent Literature 2). In this technique, a groove is formed outside the hole of a gear to reduce the thickness of the edge portion of the hole, and another component is fitted into the groove to locally deform the edge portion of the hole inwardly, so that the pinion inserted into the hole is fixed.
A technique of holding a shaft in a gear has been also proposed (see, e.g., Patent Literature 1). In this technique, a thin elastic structure extending toward an inside of a hole is formed in the gear, and the shaft is inserted in the shaft center direction with the elastic structure being elastically deformed, so that the shaft is held by a restoring force of the elastic structure.